Heretofore, in fluid flow systems wherein it was desired to restrict the flow in a line, it was common practice by the prior art to employ a simple fixed area flow restrictor, where possible, for such reasons as reduction or elimination of moving parts, the ability for exact sizing, simplicity and comparatively low cost. However, this approach by the prior art, usually required undesirable compromises when in comparison substantially improved system efficiency would result if a practical, low-cost, variable size flow restriction were available.
For example, in automotive air conditioning systems, it has been common practice in the prior art to employ a fixed restriction, such as an orifice or capillary tube in the high-pressure liquid line (conduit) between the condenser and evaporator. The refrigerant flow is primarily dependent on the inlet pressure at the orifice and the amount of liquid subcooling or quality. Quality, here, is defined as the percentage of gas in the refrigerant at the orifice inlet. Since the orifice tube flow is sonic, or "choked", suction pressure generally is not a factor in determining flow during normal system operation. Increasing inlet pressure or subcooling increases flow while increasing gas at the inlet decreases flow. These characteristics make the fixed orifice tube or capillary tube self regulating and produces adequate performance in a wide range of conditions encountered in normal automotive air conditioner operation. However, since the introduction of this type of refrigerant flow control in automobiles in 1973, efforts have continued to develop a viable variable orifice valve in an effort to improve its performance during periods of idle and low speed vehicular engine operation. Current systems use an orifice tube 1.5 inches (3.81 cm.) long by 0.060 in. (1.524 mm.) to 0.072 in. (1.83 mm.) in diameter. This size generally provides good highway speed performance at reasonable compressor head pressures.
Using an orifice tube having a diameter in the range of 0.040 in. (1.016 mm.) to 0.050 in. (1.270 mm) at idle has been shown to increase air conditioner discharge air performance by 7.degree. F. while reducing compressor power requirements by 10.0% at certain high ambient conditions. This as compared to the larger size.
To fully understand why this occurs consider that the orifice tube is sized for reasonable head pressures at high speed high load engine conditions. At these conditions (100.degree. F. to 110.degree. F. ambient) compressor discharge pressure at vehicular highway speeds is on the order of 250.0 psig. Subcooling is in the range of 15.degree. F. and refrigerant flow is 10.0 lbs. per minute of R-12 (Dichlorodifluoro Methane) or R-134A (Tetrafluord Ethane). As the vehicle is then idled condenser airflow is decreased substantially and head pressure rises to the range of 350.0 psig. Because the compressor is pumping much less refrigerant (5.0 lbs. per minute) the orifice tube must only produce this flow to again balance. This is accomplished by the condenser not condensing all of the gas entering it from the compressor. This uncondensed gas enters the orifice inlet and, as discussed previously, reduces flow in this case to 5.0 lbs. per minute and the refrigeration cycle goes from an efficient subcooled cycle to an inefficient quality cycle. A thermostatic expansion valve cycle would be a saturated liquid cycle and much more efficient. It is an established fact that subcooling increases system capacity by approximately 1.0% per degree of subcooling at no increase in compressor flow requirements while quality significantly decreases performance. FIG. 80 illustrates this more explicitly.
In FIG. 80, note the gain in evaporator capacity for the subcooled cycle versus the loss for the quality cycle. This is per quantity of refrigerant circulated. A too large of orifice tube results in the less efficient quality cycle occurring at low speed and idle conditions in an automobile system.
By reducing orifice size at the idle condition liquid refrigerant will back up in the condenser and if backed up enough subcooling will result and the performance can exceed the TXV saturated cycle. (Thermostatic Expansion Valves, commonly called TXV valves, control the flow rate of liquid refrigerant entering the evaporator as a function of the temperature of the refrigerant gas leaving the compressor. Some vehicles still use this relatively expensive type of expansion device since it exhibits performance advantages at low speed and idle over the fixed area orifice tube.) The back-up must be designed so as not to exceed design head pressures as condensing surface is lost with this back-up. If this small orifice were then used at the high load high speed condition more refrigerant would be required in the system as more back up of liquid occurs until enough subcooling and head pressure are available to again flow enough to satisfy evaporator requirements. Unfortunately the head pressure rises into the range of 300.0 to 400.0 psi. substantially reducing compressor life. For these reasons it is imperative that a successful variable flow orifice not be engaged into the minimum flow area at high speed high load conditions.
If the refrigerant charge is not increased at small orifice-high speed-high load then the head pressure will rise significantly but not as high as the above but the evaporator will be starved. This results in high refrigerant superheat in and out of the compressor again reducing compressor life along with an increase in evaporator out air temperature. From the above it can be seen that a variable orifice would be very desirable for increased efficiency and performance.
Flow noise of the prior art fixed orifice tube has been and is a continuing problem in many automotive installations. Start-up, running and shut down noise is somewhat attenuated by use of a diffusion screen at the orifice tube outlet but is not satisfactory in many applications. This noise is principally a function of the quality of the refrigerant entering the expansion device.
Numerous patents have been issued relating to two stage or variable flow orifices which utilize pressure differential to actuate the device. U.S. Pat. Nos. 4,375,228 and 5,081,847 teach that a 175 lb differential between suction and head is a desirable trigger point (R-12). This differential is much too low in certain soak and cool down conditions and would result in the small orifice staying in at the high load-high speed condition. Also, the hysteresis of these designs is so great that once triggered the small orifice would stay in until the system pressures were almost equalized. Again, compressor life would be reduced due to the significantly higher pressures and gas temperatures.
Two other U.S. Pat. Nos. 4,951,478 and 5,170,638 use a sliding O-ring seal. The small size debris circulating in the system and the long shut down period possible in automotive air conditioning systems could lead to sticking of this seal. In fact tight tolerance slideable fits could lead to sticking due to dirt accumulation on these surfaces. The patents mentioned above use this type of fit. A sliding O-ring seal would also undesirably increase hysteresis and be susceptible to wear.
It is noteworthy that to date there is no commercially available variable flow orifice valve which is pressure actuated for refrigeration applications in spite of almost twenty-five years of developmental effort.
An object of the invention is to provide a variable flow orifice valve assembly which is effective and reliable in its operation and low cost to manufacture and designed as to be inserted into a fluid line or conduit.
Another object of the invention is the construction of a variable flow orifice valve assembly which will engage the minimum orifice size at sufficiently high pressure differential to preclude staying in at high speed high load conditions.
A further object of the invention is the construction of a variable flow orifice valve assembly which will have a middle range orifice size infinitely variable or constant between maximum and minimum sizes small enough to obtain significant performance gains at low speed and idle but large enough to not have adverse effects at high speed high load conditions.
Another object of the invention is the construction of a variable flow orifice valve assembly which will possess or exhibit very low actuating hysteresis (in the range of 0.0 to 20.0 psi.)
Another object of the invention is the construction of a variable flow orifice valve assembly which will engage the minimum orifice size at significantly higher pressure differential during high load high flow conditions as compared to idle and low speed low refrigerant flow conditions.
Another object of the invention is the construction of a variable flow orifice valve assembly which will have internal flow passages large enough to pass therethrough debris which may have been permitted to flow into such flow passages by said inlet screen.
A further object of the invention is the construction of a variable flow orifice valve assembly which will re-open to the maximum or larger than minimum orifice size in case of spring failure or abnormally high pressure differential.
Another object of the invention is to construct a variable flow orifice valve assembly which will be commercially acceptable for low noise.
A still further object of the invention is to construct a variable flow orifice valve assembly which will have the spring isolated from the majority of refrigerant flow to preclude undesirable spring vibration caused by such flow.